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GLIESE 687 B—A NEPTUNE-MASS PLANET ORBITING a NEARBY RED DWARF

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GLIESE 687 B—A NEPTUNE-MASS PLANET ORBITING a NEARBY RED DWARF The Astrophysical Journal, 789:114 (14pp), 2014 July 10 doi:10.1088/0004-637X/789/2/114 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE LICK–CARNEGIE EXOPLANET SURVEY: GLIESE 687 b—A NEPTUNE-MASS PLANET ORBITING A NEARBY RED DWARF Jennifer Burt1, Steven S. Vogt1, R. Paul Butler2, Russell Hanson1, Stefano Meschiari3, Eugenio J. Rivera1, Gregory W. Henry4, and Gregory Laughlin1 1 UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA 2 Department of Terrestrial Magnetism, Carnegie Institute of Washington, Washington, DC 20015, USA 3 McDonald Observatory, University of Texas at Austin, Austin, TX 78752, USA 4 Center of Excellence in Information Systems, Tennessee State University, Nashville, TN 37209, USA Received 2014 February 24; accepted 2014 May 8; published 2014 June 20 ABSTRACT Precision radial velocities from the Automated Planet Finder (APF) and Keck/HIRES reveal an M sin(i) = 18 ± 2 M⊕ planet orbiting the nearby M3V star GJ 687. This planet has an orbital period P = 38.14 days and a low orbital eccentricity. Our Stromgren¨ b and y photometry of the host star suggests a stellar rotation signature with a period of P = 60 days. The star is somewhat chromospherically active, with a spot filling factor estimated to be several percent. The rotationally induced 60 day signal, however, is well separated from the period of the radial velocity variations, instilling confidence in the interpretation of a Keplerian origin for the observed velocity variations. Although GJ 687 b produces relatively little specific interest in connection with its individual properties, a compelling case can be argued that it is worthy of remark as an eminently typical, yet at a distance of 4.52 pc, a very nearby representative of the galactic planetary census. The detection of GJ 687 b indicates that the APF telescope is well suited to the discovery of low-mass planets orbiting low-mass stars in the as yet relatively un-surveyed region of the sky near the north celestial pole. Key words: planets and satellites: detection – planets and satellites: gaseous planets – stars: individual (GL687) Online-only material: color figures 1. INTRODUCTION definitive statement that is couched in planetary masses as well as in planetary radii. Figure 1 shows the current distribution The Copernican principle implies that the Earth, and, by of reported planets and planetary candidates orbiting primaries extension, the solar system, do not hold a central or specifically with M < 0.6 M, which we adopt as the functional border favored position. This viewpoint is related to the so-called between “M-type” stars and “K-type stars.” mediocrity principle (Kukla 2010), which notes that an item The census of low-mass planets orbiting low-mass primaries drawn at random is more likely to come from a heavily populated can be accessed using a variety of techniques. For objects category than one which is sparsely populated. near the bottom of the main sequence, it appears that transit These principles, however, have not had particularly apparent photometry from either ground (Charbonneau 2010) or space success when applied in the context of extrasolar planets. Mayor (Triaud et al. 2013) offer the best prospects for planetary et al. (2009) used their high precision Doppler survey data to discovery and characterization. For early to mid M-type dwarfs, deduce that of order 50% (or more) of the chromospherically there is a large enough population of sufficiently bright primaries quiet main-sequence dwarf stars in the solar neighborhood are that precise Doppler detection (see, e.g., Rivera et al. 2010) accompanied by a planet (and in many cases, by multiple can play a lead role. For the past decade, we have had a planets) with M sin(i) 30 M⊕, and orbital periods of P< sample of ∼160 nearby, photometrically quiet M-type stars 100 days. Taken strictly at face value, this result implies that our under precision radial velocity (RV) surveillance with the Keck own solar system, which contains nothing interior to Mercury’s telescope and its HIRES spectrometer. In recent months, this P = 88 day orbit, did not participate in the galaxy’s dominant survey has been supplemented by data from the Automated mode of planet formation. Yet the eight planets of the solar Planet Finder (APF) telescope (Vogt et al. 2014b). Here, we system have provided, and continue to provide, the de facto present 16.6 yr of Doppler velocity measurements for the nearby template for most discussions of planet formation. M3 dwarf GJ 687 (including 122 velocity measurements from Indeed, where extrasolar planets are concerned, M dwarfs Keck, 20 velocity measurements from the APF, and 5 velocity and mediocrity appear to be effectively synonymous. Recent measurements made with the Hobby–Eberly Telescope (HET)) observational results suggest that low-mass planets orbiting low- and we report the detection of the exoplanet that they imply. We mass primaries are by no means rare. Numerous examples of use this discovery of what is a highly archetypal representative planets with Mp < 30 M⊕ and M dwarf primaries have been of a planet in the Milky Way—in terms of its parent star, its reported by the Doppler surveys (e.g., Butler et al. 2004; Mayor planetary mass, and its orbital period—to motivate a larger et al. 2009, and many others), and the Kepler Mission has discussion of the frequency of occurrence, physical properties, indicated that small planets are frequent companions to low- and detectability of low-mass planets orbiting M-type stars. mass stars. For example, Dressing & Charbonneau (2013) report The plan for this paper is as follows. In Section 2, we describe that among dwarf stars with Teff < 4000 K, the occurrence rate of the physical and spectroscopic properties of the red dwarf host = +0.04 star Gliese 687. In Section 3, we describe our RV observations of 0.5 R⊕ <Rp < 4 R⊕ planets with P<50 days is N 0.9−0.03 planets per star. Improved statistics, however, are required for a this star. In Section 4 we describe our Keplerian model for these 1 The Astrophysical Journal, 789:114 (14pp), 2014 July 10 Burt et al. Figure 1. Population diagram for currently known extrasolar planets orbiting stars with reported masses Mstar < 0.6 M. Green circles: planets securely Figure 2. H-R diagram with GJ 687’s position indicated as a small open circle. detected by the radial velocity method (either with or without photometric Absolute magnitudes, M, are estimated from V band apparent magnitudes and = transits). Red circles: the regular satellites of the Jovian planets in the solar Hipparcos distances using M V +5log10(d/10 pc). All 956 stars in our system. Gray circles: Kepler candidates and objects of interest. Radii for these catalog of radial velocity measurements (for which more than 20 Doppler candidate planets, as reported in (Batalha et al. 2013), have been converted measurements exist) are shown, color-coded by their B−V values, with point to masses assuming M/M⊕ = (R/R⊕)2.06 (Lissauer et al. 2011), which is areas sized according to the number of observations taken. obtained by fitting the masses and radii of the solar system planets bounded in (A color version of this figure is available in the online journal.) mass by Venus and Saturn, which may be a rather naive transformation given the startling range of observed radii for planets with masses between Earth and Uranus. Venus, Earth, and Jupiter are indicated on the diagram for comparison purposes. Data are from www.exoplanets.org, accessed 2014 December 1. (A color version of this figure is available in the online journal.) observations, along with an analysis that assesses our confidence in the detection. In Section 5, we describe our photometric time series data for the star, which aids in the validation of the planet by ruling out spot-modulated interpretations of the Doppler variations. In Section 6, we discuss the ongoing refinement of the planet–metallicity correlation for low-mass primaries, in Section 7, we discuss the overall statistics that have emerged from more than 15 yr of precision Doppler observations of M dwarf stars with the Keck Telescope, and in Section 8 we conclude with an overview that evaluates the important future role of the APF telescope in precision velocimetry of nearby, low-mass stars. 2. GJ 687 STELLAR PARAMETERS Figure 3. Average value of the S-index against the standard deviation of the S-index for all the stars in the Lick–Carnegie database. Stars with M < 0.6 M ◦ Gliese 687 (LHS 450, BD+68 946) lies at a distance, are colored red. GJ 687 is shown as an orange circle in the midst of this d = 4.5 pc, is the 39th nearest known stellar system, and is population, showing that it is a somewhat active star. The areas subtended by the closest star north of +60◦ declination. Figure 2 indicates the individual points are, in all cases, proportional to the number of Doppler velocity observations that we have collected of the star (with systems above an GJ 687’s position in the color–magnitude diagram (CMD) for upper bound of 250 observations receiving the same point size). stars in the Lick–Carnegie Survey’s database of Keck obser- (A color version of this figure is available in the online journal.) vations. Due to its proximity and its brightness (V = 9.15, Ks = 4.548), Gliese 687 has been heavily studied, and in par- ticular, the CHARA Array has recently been used to obtain 3. RADIAL VELOCITY OBSERVATIONS direct interferometric angular diameter measurements for the star.
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